Answer:
This is because the air at the surface has a high density due the shear weight of the other particles above. This however changes as you increase elevation as it means that there are less particles above and so would decrease the density.
This can affect humans as it means with every breath we would take in less oxygen (O2) this intern would mean that our bodies would be deprived of oxygen. Also that many of us have become accustomed to the atmospheric pressure from the surface and so many mountain climbers need to become accustomed to such altitudes.
An example of this being used in our lives is on the pressurisation systems on aircraft. This is because at the altitudes the aircraft fly at the air density is decreased and so the aircraft must be pressurised to help maintain the air pressure and density as on the surface